Belly is a nearly 16-year-old girl who measures her entire life by the summers she spends at Cousins Beach. Having grown up as the youngest and only girl among her brother and the Fisher boys, she consistently feels like an outsider. This summer, she notices a physical maturation in herself that finally catches the boys' attention. She desperately wants to be seen as an adult rather than a little sister.

Conrad is the older Fisher brother and the long-time object of Belly's affection. He recently quit football and appears more brooding and closed-off than in previous summers. He often masks his true emotions and keeps those around him at a distance. Despite his aloof exterior, he maintains an undeniable, magnetic presence in the beach house.

Jeremiah is the younger, more outgoing Fisher brother. He has always been a reliable friend to Belly, often joining in on childhood teasing but also showing genuine care by teaching her to drive a stick shift. This summer, his physical growth from football and shifting demeanor suggest a changing dynamic in his normally platonic relationship with Belly.

Laurel is Belly and Steven's mother, a practical woman who recently finalized a divorce from her husband. She is lifelong best friends with Susannah Fisher and views the beach house as a shared sanctuary. Her calm, unemotional demeanor sometimes causes friction with her daughter, who wishes Laurel would express her feelings more openly.

Susannah is Conrad and Jeremiah's mother and the owner of the Cousins Beach house. Vivacious and deeply intuitive, she has always predicted that Belly will end up with one of her boys. She acts as a warm, encouraging bridge between Belly and Laurel. She spends more time resting in her room this summer, harboring private struggles.

Steven is Belly's older brother who frequently teases her and excludes her from his activities with the Fisher boys. His presence at the beach house represents the familiar status quo of their childhood. His planned early departure for a college tour serves as a jarring reminder to Belly that their lives are changing.

Cam is an intelligent, straight-edge boy who becomes Belly's summer fling. He remembers Belly from years ago, recognizing her beauty even before she physically matured. His genuine affection offers Belly her first taste of summer romance, though his presence creates friction with the Fisher brothers.

Taylor is Belly's best friend from home who has more experience with dating. When Taylor visits the beach house, her flirtatious nature and immediate pursuit of the Fisher brothers trigger Belly's insecurities. Her presence highlights the differences between the two girls and complicates their friendship.

Mr. Fisher is Susannah's husband and the father of Conrad and Jeremiah. Though usually aloof, his presence demands respect, and Conrad has historically idolized him. His unusual absence this summer contributes to the shifting dynamics and underlying tensions within the Fisher family.

Belly and Steven's father, who recently divorced Laurel. He attempts to make his new, dingy apartment welcoming for his children, but Belly resents the changes he represents and prefers the stability of Cousins Beach to the reality of her broken family.

Clay is an older local boy who hangs out with the Fisher brothers. He is the first person to treat Belly like one of the older crowd by inviting her to his Fourth of July bonfire party, signaling her transition into young adulthood.

Nicole is a girl Conrad invites over to the house to watch a movie, sparking immediate jealousy in Belly. Her presence acts as a catalyst for Belly to question her own lingering feelings for Conrad despite her ongoing relationship with Cam.